This invention relates to an apparatus and method for the compact, selectively controlled and metered introduction of a medical fluid, such as a drug, through the tympanic membrane and into the middle ear of a patient.
It is desirable in certain circumstances to administer medications or other medical fluids into the middle ear through the tympanic membrane. Intratympanic perfusion of drugs for treatment of inner ear conditions was popularized the 1970's in Europe with the treatment of Meniere's disease with intratympanic aminoglycoside antibiotics. This route of administration has since gained wider utilization in the treatment of many other ear conditions as well, including tinnitus, sudden hearing loss, and various forms of labyrinthine dysfunction. Medications typically used include aminoglycosides, corticosteroids and local anesthetics. Anticipated delivery of other medications by this route has undergone widespread discussion.
Intratympanic delivery of drugs has been accomplished in the past principally by making a small incision in the anesthetized tympanic membrane (ear drum), inserting a needle or catheter into the middle ear, infusing the drug in liquid form and allowing it to be absorbed into the inner ear, probably mainly by way of the round window membrane. Other methods have included placing an incision or implanted tube in the tympanic membrane and then having the patient self-dispense the drug into the external ear canal whereby it is intended to pass through the opening into the middle ear, and thence the inner ear. This has the disadvantage that infectious debris can be carried into the middle ear from the external canal, with the risk of creating a middle ear infection, and passage of the liquid into the middle ear is inhibited by the surface tension of the liquid. These problems have been partially solved by inserting a wick between the external ear canal and the middle ear, as in U.S. Pat. No. 6,120,484 to Silverstein. But this method has the disadvantages of possible patient noncompliance, errors in following directions, confusion of medications, failure of some or all of the instilled drops to reach the wick and chronic perforations due to the extended use of the wick.
U.S. Pat. No. 5,474,529 to Arenburg describes a multi-functional inner ear treatment and diagnostic system, using a two channel catheter that leads to a small reservoir placed in the round window niche and connected with the round window membrane via small openings through which the drug is allowed to diffuse. Implantation is difficult, and the large catheter often leaves a defect in the tympanic membrane.
Other examples of apparatus and methods for accomplishing intratympanic drug delivery are described, for example, in U.S. Pat. Nos. 4,034,759, 5,421,818, 5,474,529, 5,476,446, 6,368,315, 6,440,102 and 6,685,697.
Recent studies have shown that there is a blood-labyrinth barrier similar to the blood-brain barrier, such that very little of most medications delivered systemically (oral, IV, etc.) is transported to the inner ear. Thus, to accomplish a therapeutic concentration of the medication within the inner ear when delivered via the systemic route, high concentrations of the medication over sustained periods of time are necessary, increasing the risk of systemic side effects.
On the other hand, if a solution containing the drug molecules is placed into the middle ear and is allowed to remain for a period of time, a small portion of the molecules will be absorbed into the inner ear, probably mainly by diffusion through the round window membrane. The amount of absorption of the drug molecules through the round window, and hence the dose of the drug reaching the inner ear structures, is proportional to the concentration of the drug in contact with the round window membrane, and the time the drug remains in contact with the round window membrane at said concentration.
The middle ear cavity can hold approximately 0.5 cc of fluid. Its outer surface is lined by a mucous membrane, which absorbs medication molecules from the middle ear. If a solution (perfusate) containing medication is thus placed in the middle ear cavity, the molecules of that medication in the solution will diffuse over time into the surrounding tissues, including the round window membrane. The round window represents only a small proportion (less than 2%) of surface area of the surrounding tissues. Therefore only a small portion of the molecules of drug will diffuse through the round window into the inner ear. On the other hand, the volume of the inner ear is only about 1 cc, so that relatively few molecules of medication are needed to obtain a therapeutic concentration in the inner ear. Molecules of drug diffuse out of the solution into the surrounding tissues so that concentration of drug in the solution becomes less with time, following an asymptotic curve. The applicant's studies indicate that the half-life of drug molecules (i.e., time until one-half of the quantity of drug is depleted) in the solution in the middle ear is approximately 5 minutes. Thus, if the typical protocol for single injection is followed, and 0.5 cc of a drug is infused into the middle ear and allowed to remain for 30 minutes, the concentration of the drug in the middle ear becomes quite low during last 15-20 minutes.
What is desired is an apparatus that is easily used, is capable of controlled administration of fluids into the ear, is stable and comfortable, and which is easily inserted and removed with minimal damage to the tympanic membrane.